This invention relates to hybrid fuel cell and turbine generator systems, and in particular, to electrical generation for hybrid fuel cell power plants.
A fuel cell is a device that directly converts chemical energy in the form of a fuel into electrical energy by way of an electrochemical reaction. In general, like a battery, a fuel cell includes a negative electrode or anode and a positive electrode or cathode separated by an electrolyte that serves to conduct electrically charged ions between them. In contrast to a battery, however, a fuel cell will continue to produce electric power as long as fuel and oxidant are supplied to the anode and cathode, respectively. In order to produce a useful amount of power, individual fuel cells are typically arranged in stacked relationship in series.
A fuel cell may be combined with a heat engine such as a turbine generator to produce a high efficiency system, commonly called a hybrid system. In a conventional hybrid system, a fuel cell is typically situated in the position normally occupied by the combustor of the turbine generator so that air compressed by the turbine generator compressor section is heated and then sent to the fuel cell cathode section. In another hybrid system, a heat engine in the form of a turbine generator is bottomed with a fuel cell so that the heated air discharged from the gas turbine is delivered to the cathode section of the fuel cell. For example, U.S. Pat. No. 6,365,290, assigned to the same assignee herein, discloses such a hybrid fuel cell/gas turbine system, in which waste heat from the fuel cell is used by a heat recovery unit to operate the heat engine cycle, and the system is arranged such that the compressed oxidant gas, heated in the heat recovery unit and by a high temperature heat exchanger, is expanded in the expansion cycle of the heat engine. It is then passed through an oxidizer (also commonly called combustor or burner), which also receives the anode exhaust, passed through the heat exchanger and the resultant gas delivered to the cathode section of the fuel cell.
Despite the efficiency benefits provided by current hybrid fuel cell systems, such as those described above, these systems have had certain limitations. Specifically, the size and power output of the turbine generator has to be matched to the size and power output of the fuel cell power plant and the turbine generator has to be synchronized to the electric power grid. For example, in a sub-MW hybrid fuel cell plant, a 60 KW microturbine is suitable for use as the turbine generator. Such hybrid system uses an inverter-based power conditioner to rectify the high frequency output of the microturbine generator and to provide a 60 Hz output so as to synchronize the microturbine to the electric power grid. However, for MW-scale power plants, microturbines are usually not available in the required size range. For example, 1.8 MW DFC/T® and 3.6 MW DFC/T® power plant, both based on Molten Carbonate Fuel Cell (MCFC) stacks manufactured by the assignee herein, require turbine-generators rated at about 325 KW and 650 KW, respectively. Since microturbine generators are not available in this range, a larger synchronous generator and a commercially available, regenerative variable frequency drive have to be used in the MW-scale hybrid power plants.
There are a number of disadvantages of using such generators in the hybrid power plants, including high costs, losses in efficiency, larger footprint, requiring an additional area for the variable frequency drive enclosure, increased complexity and decreased reliability. Moreover, when the hybrid power plant uses a synchronous generator, direct connection of the synchronous generator to an electric power grid is not feasible. This is because a synchronous generator utilizes an unfired gas turbine, the mechanical power of which is determined by the fuel cell waste heat, and control of such unfired turbines requires additional complexities in the turbine design, resulting in increased cost, lower efficiencies and decreased reliability.